The invention relates to a periodically operating refrigeration machine, that is, to a thermal performance amplifier for such a machine and to a method of operating a refrigeration machine by a thermal cycle process.
It is well known to provide a refrigeration process operating according to the Stirling principle, which includes no moving mechanical parts in the cold section of the cycle. The cooler of such a machine comprises a compressor piston periodically operated at ambient temperature, a thermally isolated regenerator, a pulse tube which is also thermally insulated and which is provided at both ends with heat exchangers, and an expansion piston, which is also operated at ambient temperature. The pistons are so moved that the pulse tube experiences the following cycle:
Compression of the gas;
Moving the gas toward the expander;
Expansion of the gas;
Moving the gas toward the compressor.
A detailed analysis shows that a relatively large amount of energy is supplied to the compressor. A relatively small part thereof is re-gained in the expander. The difference is converted into heat which must be essentially removed in the area of the compressor (See also FIG. 6).
Such cooling cycles have been built in some differently modified ways. With single stage arrangements, the temperature can be reduced typically from room temperature to about 25xc2x0 K [I, II]; with two-stage arrangements, temperatures of less than 4xc2x0 K can be reached [III].
In a periodically operating refrigeration machine which includes a thermal performance amplifier based on the known pulse tube process, the thermal performance amplifier includes a compression arrangement with a first heat exchanger for transferring heat to the environment, a regenerator, a second heat exchanger supplying heat to the performance amplifier, a pulse tube, and a third heat exchanger disposed adjacent the pulse tube cooler for removing heat. The pulse tube cooler also includes a regenerator, a heat exchanger and a pulse tube, another heat exchanger and an expander, all sized for optimal operation.
The invention was arrived at by the following considerations:
If, in the heat exchanger between the regenerator and the pulse tube so much energy is added that no cooling but rather, heating above room temperature occurs, the energy to be removed at the expander is greater than the compression energy mechanically supplied to the system. A part of the heat added in the heat exchanger between the regenerator and the pulse tube and removed in the heat exchanger at the end of the pulse tube is converted to work and therefore results in an increase in the mechanical power.
The mechanical energy gained in this way is usable in the operation of a pulse tube cooler.
A refrigeration machine using this concept includes a thermal power amplifier and a pulse tube cooler arranged at the exit of the thermal power amplifier which, accordingly, are arranged in series.
The thermal performance amplifier comprises a compressor arrangement to which a first heat exchanger is connected which transfers heat to the environment. A regenerator is connected to the heat exchanger. At the other end, a second heat exchanger is provided by way of which heat is supplied to the performance amplifier. This heat exchanger is therefore termed a heater. The pulse tube of the power amplifier is connected to the heater and, at the opposite end, to a heat exchanger, which discharges heat from the pulse tube. The pulse tube cooler is connected to the last mentioned heat exchanger. In this arrangement, the last heat exchanger of the power amplifier is the first heat exchanger of the pulse tube cooler. Between the regenerator and the pulse tube of the pulse tube cooler, there is the heat exchanger, which forms the usable refrigeration zone. Finally, the pulse tube includes a last heat exchanger followed by an expansion device coupled thereto.
There are different operational variants of pulse tube coolers [I-III].
There are two variants with movable parts:
The Stirling process with a piston expander and the Stirling process with a passive expander,
and there are two variants which have no movable parts:
The Gifford-McMahon operating system with a high and a low pressure reservoir, which are both connected to a regenerator, each by way of a supply line including a valve and a passive expander and finally the Gifford-McMahon-operating system with a compression device and with a controllable valve arranged in the communication line from the high and the low pressure reservoirs, the valve controlled expander, to the pulse tube.
The pulse tube amplifier may be heated electrically, but other heat sources such as solar heat or combustion heat can be utilized like with the Sterling motor. In this case, the cooler can be operated with a further reduced need for primary energy.
With the invention, the following advantages are achieved:
The efficiency is improved so that less primary energy is consumed for the same refrigeration effect;
The manufacture of the cooler is relatively inexpensive;
in comparison with a mechanical compressor, a pulse tube amplifier can be manufactured very inexpensively, the additional expenses are compensated for by the need for only a small compressor;
the operating costs are relatively low;
maintenance costs are low, the pulse tube amplifier itself needs no maintenance. The additional components needed for the pulse tube cooler require only relatively small components such as a compressor and valves which need to be serviced or exchanged periodically, but they are relatively small and therefore relatively inexpensive.
Below, the invention will be described in greater detail on the basis of the accompanying drawings: